This invention relates generally to fabrication techniques for field-effect transistors, and more particularly, to the frabrication of non-coplanar field-effect transistors. There is a need to develop a millimeter-wave transistor for operation at frequencies of 40 gigahertz (GHz) and higher.
The technique proposed in the cross-referenced application was to etch a via hole through the substrate of device, and to align the source region of the device with the via hole using electron beam lithography (EBL) methods. This technique, which provides a generally satisfactory solution to the problem of aligning souce and gate regions in an opposed gate-source FET, suffers from one significant disadvantage. The etching of multiple via holes in a substrate wafer is hardly ever a uniform process, since the substrate thickness, etching rate, and other parameters may vary. Moreover, a rectangular via hole will typically be somewhat convex at the area of contact with the source region, like the cross section of a meniscus lens. Consequently, the incremental source resistance at the shorter ends of the meniscus will be higher than at the center, and there will be a longer transit time associated with these shorter end regions.
Accordingly, although these drawbacks can be avoided by appropriately customizing a single FET structure, it would be highly desirable to provide an improved technique in which the drawbacks were eliminated, and mass production of the devices thereby facilitated. This invention represents an alternative to the approach proposed in the cross-referenced application.